Dedifferentiated liposarcoma with abrupt transition of low-grade and high-grade dedifferentiated components: A case report

Dedifferentiated liposarcoma (DDLPS) is a unique subtype of liposarcoma, which has obvious histological heterogeneity. In affected patients, the condition typically manifests as the dedifferentiation of high-grade histological morphology, but it may also manifest as the dedifferentiation of low-grade histological morphology. In some cases, unique histological or immunophenotypic characteristics are observed. We describe, herein, a rare case of dedifferentiated liposarcoma, in which the high-grade and low-grade dedifferentiated components coexisted with a relatively sharp transition in pathology. A Physical examination revealed a mobile large left abdominal mass, Magnetic resonance imaging (MRI) scan showed a huge mass with typical fat components and the non-fatty nodule in the left retroperitoneal cavity. After laparotomy, histologic analysis of the specimens could nd the atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDLPS) and DDLPS components. Fluorescence in situ hybridization (FISH) analysis suggested the presence of MDM2 gene amplication. These ndings supported a diagnosis of DDLPS. low-grade dedifferentiation may be a precursor to high-grade dedifferentiation. MRI images cannot distinguish the two components.


Introduction
Liposarcoma is the single most common soft tissue sarcoma, accounting for 20-35% of soft tissue sarcomas [1,2] . Although atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDLPS) and dedifferentiated liposarcoma (DDLPS) have similar genetic characteristics, WDLPS and DDLPS represent two ends of the histologic and behavioral spectrum for a single disease entity [2,3] . Histopathologically, DDLPS may show a markedly heterogeneous morphology. In most patients, the condition manifests as the dedifferentiation of high-grade histological morphology, similar to undifferentiated pleomorphic sarcoma or high-grade myxo brosarcoma. In some cases, the condition may manifest as the dedifferentiation of low-grade histological morphology or mixed high-grade and low-grade histological morphology [4][5][6] . Low-grade DDLPS may be similar in histology to low-grade myo broblastic sarcoma, bromatosis, in ammatory myo broblastic tumor, gastrointestinal stromal tumor (GIST), or solitary brous tumor [4,5,7] . Distinguishing the various histological types of liposarcoma is not always easy, even for an experienced pathologist, especially when only a few samples are available. In this manuscript, we retrospectively analyze the pathological and imaging ndings of a rare case of DDLPS with mixed high-grade and low-grade dedifferentiated histological features with multiple focal regions of a sudden transition. We also review the relevant literature.

Case Report
The patient was a 69-year-old woman, who was admitted to the emergency department of our hospital with severe abdominal pain lasting 1 hour in February 2020. The patient accidentally discovered a mass in the left middle abdomen 4 months prior. The patient had begun to have abdominal cramps repeatedly over the preceding 3 months. The abdominal pain was intermittent, and it relieved spontaneously after discharging a large amount of watery stool. On physical examination, a very large soft mass could be palpated in the left abdomen. Laboratory ndings were considered as normal, and tumor markers (carcinoembryonic antigen and CA19-9) were within normal limits.

Radiologic ndings
Magnetic resonance imaging (MRI) examination showed an 8 cm × 13.3 cm × 20.9 cm mass in the left retroperitoneal cavity. The MRI signal intensity for most of the mass was hyperintense on T1-weighted images and T2-weighted images, with drop-out on MRI fat-suppressed sequences images. (Fig.1a-b) A 7.8 cm × 10.6 cm × 11.2 cm solid nodule was seen in the lesion with a heterogeneous signal. T2-weighted images show mixed-intensity signal. T1-weighted images showed iso-intensity signal, without signal dropout on MRI fat-suppressed sequenced images. The apparent diffusion coe cient (ADC) map showed an irregular low-intensity signal at the edge of the nodule, with an ADC value of 0.913×10 -3 . (Fig.1c) On contrast-enhanced images, the non-fatty nodule showed irregular peritumoral enhancement and no enhancement in the central region. (Fig.1d)

Surgical and Pathologic features
During laparotomy, the well-circumscribed, lobulated mass, which was located in the left retroperitoneum and about 25 cm × 20 cm × 22 cm in size, underwent complete excision. The upper part of the tumor was very hard and adhered to the mesocolon of the descending colon; a large amount of brown-yellow fat-like tissue was seen in the lower part of the tumor.
Grossly, the size of the tumor was approximately 20 cm × 16 cm × 7 cm, and the size of the grayishyellow fatty mass was about 9 cm × 8 cm × 7 cm. A round nodule with a complete capsule was seen adjacent to the fatty mass, which was about 11 cm × 9.5 cm × 7 cm in size. A grayish-yellow necrotic area (about 8 × 6 × 5 cm) was seen in the nodule's center, surrounded by a crescent-shaped grayish-white and grayish-brown tumor. There was a clear boundary between brown tumor foci near the capsule and other gray-white or sh esh-colored tumor foci. (Fig.2) Microscopically, the grayish-yellow lipoid nodule was ALT/WDLPS, and the solid nodule was DDLPS.
( Fig.3a-b) DDLPS showed two different histological types and grades. Most of the solid nodule were highgrade pleomorphic undifferentiated sarcomas with extensive tumor necrosis, the tumor cells had obvious atypia and active mitotic images (about 50/50 high-power microscopic eld; HPF). The low-grade DDLPS were located near the capsule of the solid nodule, histologically, they were in ammatory myo broblastic tumor-like (Fig.3c) and bromatosis-like features. (Fig.3e), most of the tumor cells were spindle-shaped, had slight atypia, with pleomorphic tumor cells occasionally seen. The average mitotic images were about 3/50 HPF, and no tumor necrosis was identi ed. The low-grade DDLPS with multiple foci can be seen, which suddenly transits to the high-grade DDLPS. (Fig.3d-f) Immunohistochemical staining showed that p16, MDM2, and CDK4 were diffusely expressed in ALT/WDLPS and DDLPS (including high-grade dedifferentiation and low-grade dedifferentiation), and smooth muscle actin (SMA) and CD34 were locally expressed in the low-grade DDLPS. Both components were negative for DOG1, c-KIT, desmin, and S100 protein. Ki67 is about 60% in high-grade DDLPS and 8% in low-grade DDLPS. Fluorescence in situ hybridization (FISH) analysis suggested the presence of MDM2 gene ampli cation. (Fig. 4) These ndings supported a diagnosis of DDLPS.
The patient recovered well and was discharged from the hospital on the 7th day after surgery. Followedup was performed regularly, and there was no sign of local recurrence or distant metastasis during a 12 months follow-up.

Discussion
According to the World Health Organization (WHO) classi cation standards, the de nition of DDLPS is a bone and soft tissue tumor or ALT / WDLPS that has dedifferentiated into a different degree of sarcoma at the same time or before/after the development of ALT / WDLPS [2,8] . Dedifferentiated areas usually consist of undifferentiated pleomorphic sarcoma or spindle cell sarcoma, with high to moderate cellularity and pleomorphism. Traditionally, DDLPS are all considered high-grade sarcomas, and no further histological grading is required.
Recently published clinicopathological analyses have con rmed that DDLPS can be further divided into low-grade (equivalent to Grade 2) and high-grade DDLPS (equivalent to Grade 3) according to the French National Federation of The Centers for the Fight Against Cancer (FNCLCC) grading system. Grade 2 DDLPS occupied about 10% of all DDLPS and had a better prognosis and overall survival rate than Grade3 DDLPS [7,9,10] . In contrast to ALT / WDLPS, which has a relatively clear histological subtype, DDLPS represents a morphologically heterogeneous group. In the case described above, DDLPS was characterized by the coexistence of high-and low-grade dedifferentiated components. This manifestation is rare in clinical practice. We summarize the associated pathological features and MRI ndings to deepen the awareness of this rare type of DDLPS.
DDLPS presents most commonly in middle-aged and older adults and affects both genders equally. The condition is extremely rare in children and adolescents. The retroperitoneum is the site most frequently affected, followed by the limb and spermatic cord / paratesticular area. Rarely affected sites include the chest cavity, mediastinum, and head and neck (such as the larynx or esophagus). Due to the large space for tumor growth in the posterior peritoneal area, ALT / WDLPS in this area can grow for a long time without causing symptoms. There is therefore a high risk (about 28%) that dedifferentiation will be observed at the time of diagnosis [4,7] .
The histology of DDLPS usually includes ALT / WDLPS components that have transformed into non-fatty tumor components, and the two components are usually clearly demarcated under the microscope. The most common histological type of ALT / WDLPS in DDLPS is lipomatous and sclerotic. As seen in our case report, the retroperitoneal mass was huge and contained many WDLPS components, the dedifferentiated components were characterized by the coexistence of low-grade and high-grade dedifferentiated components.
High-grade dedifferentiation can coexist with low-grade dedifferentiation [11] , but this is rare. The pathological manifestations of this case belong to this category. In this case, the histological morphological characteristics of low-grade dedifferentiation in the specimen showed bidirectionally, some areas showed in ammatory myo broblastic tumor-like characteristics changes, while some areas showed bromatosis-like characteristics changes, and the coexistence of these two histomorphology re ected the variability and relative instability of the histological phenotype of low-grade dedifferentiation when they are formed. The low-grade dedifferentiation components accounted for only 10% of the dedifferentiated tumor and were located in the periphery of the high-grade dedifferentiation components, and the two dedifferentiated portions suddenly transition with a clear boundary. We speculated that lowgrade dedifferentiation may be a precursor to high-grade dedifferentiation, the conversion starts from the periphery in DDLPS, this manifestation has also been reported in another case report [12] . Besides, the dedifferentiation components can also show other histological morphological characteristics, such as concentric circle-like changes, even mixed distribution, etc [13] .
As mentioned above, DDLPS can show a wide spectrum of histological morphology. In addition to the common histological features similar to other types of soft tissue sarcomas described above, many structural characteristics that are di cult to describe in morphology can also be seen. If DDLPS occurring outside the retroperitoneum, it can simulate the more common soft tissue tumor in this site, thus confusing the differential diagnosis, particularly for the low-grade DDLPS. For example, the low-grade DDLPS occurring in the gastrointestinal tract may simulate the more common GIST or in ammatory brous polyp [14] , and the low-grade DDLPS arising in the paratestcular locations may be similar to the cellular angio broma, etc. This also reminds us to be vigilant and carefully understand in our daily work.
Besides, in the differential diagnosis of fatty tumors other than ALT / WDLPS, immunohistochemical staining that is positive for p16, MDM2 and CDK4 have high sensitivity and speci city for the diagnosis of DDLPS. In this case, well-differentiated liposarcoma components and dedifferentiated liposarcoma components (including high-grade dedifferentiation and low-grade dedifferentiation) all diffusely express P16, MDM2, and CDK4. However, in the differential diagnosis of DDLPS and non-fat-derived tumors, the speci city of the above three markers is insu cient. At this time, the use of FISH to detect the ampli cation of the MDM2 gene is highly speci c and sensitive for the diagnosis of DDLPS, especially when diagnosed with small biopsy specimens. The use of FISH is even more speci c and sensitive in small biopsy specimens without typical WDLPS components or low-level dedifferentiation and rare types of DDLPS [7,15] . Ampli cation of the MDM2 gene is generally considered as the gold standard for the diagnosis of ALT / WDLPS and DDLPS.
The diagnosis of DDLPS requires the existence of two components in the tumor: lipogenic WDLPS and cellular nonlipogenic sarcoma. MRI can easily be used to identify fat-derived components in tumors through the use of fat-suppressed T2 images or short tau inversion recovery (STIR) imaging [16] . These approaches allow for the identi cation of WDLPS components in DDLPS, which is more helpful for diagnosis; however, in some cases, the WDLPS composition may go unnoticed. Because DDLPS is the conversion of WDLPS components to non-fat-derived tumor components, DDLPS lesions may lack signs of lipid characteristics on MRI.
In summary, for the diagnosis of liposarcoma, whether based on MRI or pathology, we should pay attention to typical fat components. MRI has some limitations when used for the preoperative diagnosis of liposarcoma in samples lacking fatty components or for the diagnosis of liposarcoma. The differential diagnosis for DDLPS is wide, and there are many diagnostic traps. Extensive sampling of the mass is     Gross appearance of DDLPS The cross-section of the WDLPS reveals yellow tissue, the high-grade DDLPS is sh esh-colored, and the middle necrotic area is pale yellow; the low-grade DDLPS (white arrows) is grey-white, but the local area is grayish-brown.